1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus including the fixing device.
2. Description of the Related Art
An image forming apparatus of electrophotographic scheme as forms an image on the basis of electrophotography can easily form the image having a good image quality, and hence, it is utilized widely for a copier, a printer, a facsimile equipment, a multifunctional peripheral, etc.
An electrophotographic image forming apparatus (hereinbelow, simply referred to as “image forming apparatus”) includes, for example, a photoreceptor, a charging section, an exposure section, a developing section, a transfer section and a fixing section. The image forming apparatus is an apparatus which performs a charging process, an exposure process, a development process, a transfer process and a fixation process by employing the photoreceptor and these sections, and which forms the image on a recording medium.
As the fixing unit which performs the fixation process, for example, a fixing device of heat-roller fixing type is employed. The fixing device of the heat-roller fixing type includes a fixing roller and a pressure roller. The fixing roller and the pressure roller are a pair of rollers which are brought into pressure-contact with each other. Inside at least one of the fixing roller and the pressure roller, a heat source such as halogen heater is included as a heating section.
In the fixation process, after the heat source has heated the roller pair to a predetermined temperature necessary for fixation (hereinbelow, referred to as “fixing temperature”), the recording medium on which an unfixed toner image is formed is fed to a fixing nip region which is a pressure-contact region between the fixing roller and the pressure roller. The unfixed toner image which passes through the fixing nip region, is fixed onto the recording medium such as paper under heat conducted from at least one of the fixing roller and the pressure roller, and the pressures of the fixing roller and the pressure roller. In the fixing nip region, a part through which recording medium has passed (hereinbelow, referred to as “paper sheet passing part”) has its temperature lowered, but it is heated to the fixing temperature by the heating source.
A fixing device provided in a color image forming apparatus capable of full-color printing employs an fixing roller (hereinbelow referred to as “elastic roller”) providing an elastic layer made for example of silicone rubber on a surface layer thereof. By using the elastic roller, the elastic layer provided on the surface of the elastic roller in the fixing nip region can become elastically deformed so as to conform to irregularities of the unfixed toner image, wherefore the elastic roller makes contact with the toner image so as to cover the surface of the unfixed toner image. This makes it possible to improve fixation on the unfixed color toner image that is larger in toner adherent amount than a monochromatic toner image. Moreover, by virtue of a deflection-releasing effect exerted by the elastic layer provided on the surface of the elastic roller in the fixing nip region, it is possible to provide enhanced releasability for a color toner that is more susceptible to occurrence of offset than a monochromatic toner image. Concretely, the elastic layer of the fixing roller as has been compressed by the fixing nip region and has undergone a distortion has the distortion released at the exit of the fixing nip region. At the exit of the fixing nip region, therefore, a deviation occurs between the elastic layer and the toner image. As a result, the adhesive force of the elastic layer to the toner image is decreased, and the toner releasability of the elastic layer is enhanced. Further, since the fixing nip configuration which is a configuration of the fixing roller and a pressure roller in the fixing nip region, is convexly curved in a radially-outward direction (a reverse nip configuration), it is possible to attain higher paper-stripping capability of the fixing roller and the recording medium. Thus, a self-stripping action capable of stripping of the recording medium and the fixing roller can be realized without using, for example, a stripping pawl as a stripping portion for stripping the fixing roller and the recording medium of each other, wherefore image imperfection caused by the provision of the stripping portion can be eliminated.
In such a fixing device provided in the image forming apparatus capable of full-color printing, it is necessary to make a width of a fixing nip region (hereinbelow referred to as a fixing nip width) wider in order to correspond to increase in speed. Two available methods of making the fixing nip width wider are to increase the thickness of the elastic layer of the elastic roller and to increase the diameter of the fixing roller. However, since the thermal conductivity of the elastic layer of the elastic roller is extremely low, increasing the thickness of the elastic layer of the elastic roller causes the following problems. When the elastic roller has the heating section as a conventional elastic roller inside, its warming-up time becomes longer and, furthermore, when the process speed is increased, the temperature of the fixing roller cannot stay close to a fixing temperature. Besides, when the diameter of the elastic roller is enlarged, there is the problem that the power consumption of the heating section increases.
To solve such problems, Japanese Unexamined Patent Publication JP-A 10-307496 (1998) discloses a fixing device of a belt fixing type that includes a fixing roller, a pressure roller, a heating roller having a heater for heating thereinside and a fixing belt, in which the fixing belt is supported around the fixing roller and the heating roller and the fixing roller and the pressure roller are brought into contact with each other with the fixing belt interposed therebetween. In the fixing device disclosed in JP-A 10-307496, since the fixing belt with small heat capacity is heated by the heating roller being the heating section, but the elastic layer with large heat capacity is not heated, it is possible to make a warming-up time shorter and it is not necessary to incorporate a heat section in the fixing roller, thus making it possible to provide a thick elastic layer with low hardness made of sponge rubber or the like and to secure a wider fixing nip width.
In the above-mentioned fixing device of belt fixing type, Japanese Unexamined Patent Publication JP-A 2002-333788 discloses a fixing device of a planar heat generating belt fixing type in which a planar heat generating element serves as the heating section. In the fixing device disclosed in JP-A 2002-333788, since heat capacity of the planar heat generating element is smaller than that of the heater for heating, it is possible to make the heat capacity of the heating section smaller, compared to the fixing device disclosed in JP-A 10-307496. Furthermore, since the planar heat generating element serving as the heating section abuts against the fixing belt and heat the fixing belt, a thermal response can be also improved compared to the fixing device disclosed in JP-A 10-307496 which is heated indirectly using the heater for heating, and it is possible to attain further shortening of a warming-up time and more energy saving.
Besides, with a fixing device disclosed in JP-A 10-307496, when the record media whose sizes are small relative to the maximum paper sheet passing width of the fixing device (hereinbelow, referred to as “small-size paper sheets”) are successively passed in the fixation process, a paper sheet passing part through which the small-size paper sheet passes in the fixing nip region is heated by the heating section, in correspondence with heat deprived of, and it recovers its temperature. In contrast, paper sheet non-passing parts of the fixing nip region outside the small-size paper sheet are heated by the heating section though heat is not deprived of. Consequently, there occurs the phenomenon that the temperatures of the paper sheet non-passing parts rise abnormally. At the occurrence of the phenomenon, when the paper sheet of ordinary size is passed immediately after this phenomenon, the appearances of a high temperature offset, paper wrinkles, etc. are caused by abnormal temperature rise parts. Therefore, a fixing device disclosed in JP-A 2002-333788 copes with the problem by dividing a resistance heat generating layer into a section in which heat generates in only the middle part of the layer, and a section in which heat generates in only both end parts of the layer. In this case, however, there is the problem that temperature sensors such as thermistors, and safety switches such as thermostats are required in the number of the divided sections, so that a system becomes very complicated.
The fixing device includes one of film fixing type, in addition to the one of the heat-roller fixing type. The fixing device of the film fixing type employs a fixing film which is thinner than a fixing belt, and it has a heating section arranged in a fixing nip region through the fixing film. This fixing device is used as, for example, one which is included in an image forming apparatus capable of full-color printing. In order to solve the problems as stated before, JP-A 2000-223244 discloses the fixing device of the film fixing type where the heating section is formed with a heat generation pattern having a positive temperature coefficient (PTC) characteristic and electrodes through which a current is caused to flow in the moving direction of the fixing film. According to the fixing device disclosed in JP-A 2000-223244, the abnormal temperature rises of the paper sheet non-passing parts can be prevented without making a system very complicated.
However, only a heat generating element made of a ceramic-based material such as barium titanate exists as a heat generating element having the positive temperature coefficient characteristic in which an electric resistance rises at a temperature of about 200° C. necessary for fixation as disclosed in JP-A 2000-223244, and it is usually difficult to work the ceramic heat generating element into the shape of a planar heat generating element having a curvature as indicated in JP-A 2002-333788. Accordingly, in the case of employing the ceramic heat generating element which has the positive temperature coefficient characteristic, a heating nip width which is a contact width of the heat generating element becomes narrower with respect to the moving direction of the fixing belt or the fixing film, and the heat generating element cannot sufficiently heat the fixing belt or the fixing film, so that the high operating speed of the fixing device cannot be realized.